High frequency apparatus



July 1, 1941.

HIGH

FREQUENCY APPARATUS Filed June 1, 1940' l 67 Hal; if

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Patented July 1, 1941 HIGH FREQUENCY APPARATUS James E. Kcister, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application June 1, 1940, Serial No. 338,367

Claims. (01. 179-171) My invention relates to high frequency apparatus. More specifically, it relates to a tuned circuit structure for an ultra high frequency amplifier.

In thermionic amplifiers, particularly high power push-pull amplifiers operating at ultra high frequencies of the order of 30 megacycles and upwards, such as are employed in television transmitters, resonant transmission lines are advantageously employed as tuned circuit elements. It is an object of my invention to provide an improved resonant line structure for a high power amplifier of this type.

It is another object of my invention to provide an improved transmission line structure which may advantageously constitute an integral part of an ultra high frequency power amplifier structure and perform a plurality of functions therein.

Another object of my invention is to provide a transmission line structure comprising spaced apart conductors with means to decrease the surge impedance of the line.

A further object of my invention is to provide the transmission line tank circuit structure of a high power amplifier with means to facilitate accurate adjustment of the length of the line.

Still another object of my invention is to provide an ultra high frequency tank circuit structure comprising a short-circuited resonant line with means to decrease the surge impedance of tential, not shown, may be applied in a conventional manner. The usual balanced connection between cathodes and ground is made by the conductor l5.

Both the input and output circuits of the devices I0 in Fig. 1 are illustrated as tank circuits comprising resonant transmission lines. Of course, only one of the circuits may be tuned if desired. As shown, the input tank circuit connected to the grids 12 comprises a pair of transmission line conductors IS in parallel spaced relation. T18 length of the line can be adjusted by means of the movable shorting member H.

The common grid return connection is made through an impedance [8 and a source of suitable grid bias potential, not shown, to ground. The anode tank circuit also comprises a similar pair of transmission line conductors l9 and adjustable shorting member 20. The common anode return connection is made through impedance 2| and a suitable source of anode potential, not shown, to ground. Input and output circuits 22 and 23 are illustrated as looped conductors inductively coupled to the transmission lines i6 and [9 respectively. Of course, any other types of coupling means known to the art may be employed.

A detailed explanation of the adjustment and operation of the above-described amplifier cirthe line andto reduce the physical size of the together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in which Fig. 1 is a circuit diagram of a high power push-pull amplifier for ultra high frequency operation in which my improved structure may advantageous- 1y be employed; Fig. 2 is a perspective view of an elementary transmission line structure illustrating certain principles embodied in my invention, and Fig. 3 is a side elevation, partially in section, of one embodiment of my invention.

Referring now to Fig. 1, a push-pull amplifier comprising a pair of thermionic devices In is illustrated. These may be high power triodes, for example, each having a cathode II, a grid 12, and an anode I3. The cathodes ll, illustrated as of the filamentary type, are connected to potentiometers or filament transformer secondaries I4 to which a suitable source of filament pocuit is deemed unnecessary. The proper adjustment of the respective transmission'lines, in order to tune them to resonance at the operating frequency, depends upon the electrical line characteristics and the impedances of the devices It and associated connections, as is well known. In accordance with my invention, now to be described in detail, a transmission line structure is provided particularly suited both to the electrical and mechanical requirements of such an amplifier.

The relationship between transmission line structure and its electrical characteristics will next be considered briefly. Referring now to Fig. 2, a pair of parallel tubular conductors 30 is illustrated which might comprise the transmission line conductors of one of the tank circuits of Fig. 1, for example. It is well known in transmission line theory that the surge impedance of such a line is a function both of the diameters a of the conductors employed and the distance between their centers b. If their diameters a are increased, other factors remaining constant, the surge impedance of the line is decreased; while if the distance between their centers 12 is increased, other factors remaining constant, the surge impedance is increased.

Considering for av moment the two conductors 30 alone, if the line is properly excited by a high frequency potential applied, for example, to the upper ends of the conductors 30 and if a shortcircuiting members is connected directly between the conductors at a lower point, standing electrical waves will appear on the line. The proper line length at which it exhibits the characteristics of a parallel resonant tank circuit de pends upon the surge impedance of the line and the impedance of the load to which it is connected, as mentioned previously. For example, when the line is coupled to a capacitive load it is also known that the physical length of the line may be increased if its surge impedance is decreased.

In a practical amplifier structure embodying such a line the values of the diameters a and of the spacing b are determined by other structural considerations, as will be shown presently. In accordance with my invention means are provided for decreasing the surge impedance of the line without changing these dimensions. This is accomplished by means of a pair of opposed plates 3| positioned between the conductors 33 and extending inwardly toward each other. These plates are of high electrical conductivity as, for example, copper. The outer edge of each plate conductively engages one of the conductors 3D. The inner opposed edges define a relatively narrow gap 32 which is of substantially less width than the spacing b between the conductors. An adjustable short circuiting member 33 is provided for determining the electrical length of the line, which is approximately the physical distance in Fig. 2.

The position of the short-circuiting member 33 is adjusted until the transmission line is resonant at the operating frequency. As is well known, the effect of the member 33 is to cause standing waves of voltage and current to appear upon the line. depends upon the characteristics of the load coupled to the line and upon the distance from the member 33 to the point at which the driving voltage is applied, i. e., the upper ends of the conductors 30 in Fig. 2, under the assumed conditions. The line is adjusted to resonance when the member 33 occupies a position at which the current flowing therethrough is a maximum. This also corresponds to a point of minimum voltage on the standing voltage wave. In other words, the member 33 is positioned substantially at an antinode of the current wave and at a node of the voltage wave. If the line is long enough there may be several such points, but for reasons apparent to those skilled in the art the member 33 is preferably placed at the first current antinode occurring as the member is moved downwardly from the upper ends of conductors 30.

By means of the structure just described the surge impedance of the line is greatly reduced, as will be apparent to those skilled in the art. The conductive plates 3I restrict the magnetic flux surrounding the conductors 30, so that most of the flux passing between the conductors must pass through the relatively narrow gap 32. This results in a consequent decrease in the line inductance and, therefore, a corresponding decrease in the surge impedance. Looking at it in another way, it decreases the effective spacing be- The distribution of Voltage and current tween the conductors and increases the effective diameters thereof.

A practical embodiment of my invention, as illustrated in Fig. 3, will now be described. The essential structure of a high power push-pull amplifier suitable for operation at ultra high frequencies is illustrated. This structure may be employed in a circuit such as is shown in Fig. 1, wherein corresponding elements have been given corresponding reference numerals. The amplifier comprises a pair of thermionic devices IIJ each of which is supported by a transmission line conductor I9 forming an element of the anode tank circuit. The conductors I9 comprise hollow cylinders of sufficient diameter to surround the bases of the devices I0. Any suitable means may be employed for mounting the devices ID on the conductors I9 so as to form a unitary structure therewith. As illustrated, the anode I3 of each of the devices l0 extends downwardly exteriorly thereof, forming a supporting base. The upper end of each conductor I9 is provided with a flared mounting ring 40 on which is seated a flange I3a formed on the exterior portion of the anode I3 of the device I0. A suitable clamping ring 4| in threaded engagement with the ring 40 holds the device I0 securely in position. A reentrant downwardly extending portion I3b of the anode I3 is surrounded by suitable cooling jackets 42 and 43 which direct the flow of a coolant, such as water, over the external surface of the anode to remove heat therefrom in the conventional manner. The coolant is supplied through conduits 44 extending upwardly within the conductors I9 to the water jackets 42 and 43 in the usual manner. The conductors I9 may of course themselves form one wall of the fluid cooling jackets if desired. The whole assembly is supported by a metal plate 45 on an insulator 46 to which the plate is secured by means of a stud 4! and a nut 48.

In the structure just described it will be observed that the diameter of the transmission line conductors I9 is determined by the structure of the devices III and the cooling system associated therewith. The spacing between the conductors I9 is also determined by structural requirements. Thus, the spacing between the devices I0 must be sufiicient to provide adequate cooling thereof and protection for the enclosing glass envelopes. The spacing must also be great enough to provide suflicient insulation therebetween in view of the high potentials existing between the anodes and other portions of the structure. Therefore, since the conductors I9 are necessarily spaced an appreciable distance apart, their surge impedance is of appreciable value. If an adjustable shorting member is connected directly between them to adjust the length of the transmission line, it has been found at ultra high frequencies that the line must be very short. In fact, it has been found at ultra high frequencies that it may be necessary to place such a shorting member between the flared rings 40. This is obviously impractical, both from the standpoint of making effective conductive connection between the conductors and in adjusting the electrical length thereof. A very slight movement of the shorting member on a line of such limited length results in a considerable variation of the electrical characteristics thereof. Further, the shorting member itself must be of considerable length and at ultra high frequencies its inductance may be appreciable. This has the effect of further shortening the required length of lihe and aggravates the difficulties.

In accordance with my invention these difiiculties are obviated by employing the plates 49 which are of material having high electrical conductivity, such as copper, between the conductors l9. These correspond to the plates 3i in the transmission line illustratedin Fig. 2 and previously described; They constitute opposed radial flanges or fins extending axially along the conductors l9 and defining a relatively narrow gap 50 th'erebetween. One edge of each plate conforms generally to the shapeof a conductor 19 and flared ring 40 and is-maintaine'd in good electrical contact therewith as by welding or soldering; or the plate 49 may of course form an integral part of the conductor IS. The upper end of each plate 49 is also extended upward adjacent the clamping ring 4| so as to extend substantially the full efiective length of the transmission line. The conductors I9 and plates 49 are made somewhat longer than necessary so that the line length may be adjusted by moving the shorting member 20. The member 20, which is adjustably positioned to bridge the gap 50, is relatively small and of low inductance and provides good electrical contact between the two portions of the line. In accordance with the principles previously discussed in connection with Fig. 2,

the required length of the transmission line has now been materially increased since its surge impedance has been reduced, and adjustments of the shorting member 20 may be made with much greater convenience and precision,

Since the shorting member 20 and all portions of the amplifier structure below this point are at ground radio frequency potential, the common anode connection may be made to any point on this portion of the structure, preferably at the stud 41,

The grid tank structure for the device I!) in Fig. 3 is similar to the anode circuit structure just described. The pair of transmission line conductors I6 is illustrated as diverging downwardly toward the grid connections from an upper supporting plate 52. The conductors 16 are illustrated as of hollow tubular construction and act as conduits for conveying cooling air from the pipes 53 to the upper terminal structure of the devices Iilin order to increase further the ef fective heat dissipation during operation. The plates 54 associated with the conductors l6 correspond in all respects to plates 49 associated with the conductors l9. They conform to the contour of the conductors 16, making intimate conductive connection therewith, and may also extend downwardly over the upper terminal structure of the devices l0 so as to extend substantially the entire efiective length of the transmission line, which includes the grid connections St. The conductors l6 and plates 54 are made somewhat longer than necessary so that the length of the line may be adjusted by means of the movable shorting member II. The common grid connection may be made to the shorting member 11 or to any part of the structure thereabove. If the supportin plate 52 is of conductive material, the grid connection may preferably be made thereto.

The circuit connections to the terminals 55 of the filamentary cathodes of the devices ill have been omitted from Fig. 3 to simplify the drawing as they form no part of my invention. They ma be connected to any suitable source of filament current as shown in Fig. 1.

In the illustrated embodiments of my iuvention the plates 3 I, 49 and 54 have been illustrated as extending substantially the entire length of the line. Of course, it will be appreciated that,

in some circumstances, it may be desirable to have them terminate short of one or both ends of the line. Their effectiveness is thereby somewhat reduced, but their function obviously re mains the same. Thus in some cases, the shortcircuiting members 33, 2D or Il may bridge the main transmission line conductor elements 30, I9 or IE directly and the plates 3|; 49 or 54 may be shorter than the required distance between the short-circuiting member and the end of the line to which the exciting voltage is applied.

It will be seen that I have provided an improved transmission line structure combining desirable mechanical features with highly efiicient electrical characteristics, and which is eminently suited to form an integral part of an amplifier structure.

While I have shown particular embodiments of I my invention, it will of course be understood that I do not wish to be limited thereto since modifications may be made, and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination with an ultra. high frequency amplifier comprising a pair of thermionic devices disposed in spaced relation and each having anode and grid terminals, a pair of resonant transmission line conductors connected to a like pair of said terminals and adapted and arranged to form a tuned tank circuit therewith, a pair of plates of high electrical conductivity positioned between said conductors, one edge of each plate being in conductive engagement with one of said conductors and opposing edges of said plates being in close proximity to each other and defining a relatively narrow gap therebetween, and a conductive member adjustably bridging said transmission line for adjusting the electrical length of said line substantially to resonance at the operating frequency of said amplifier.

2. In combination with an ultra high frequency amplifier structure comprising a pair of thermionic devices disposed in parallel spaced relation and each having an external anode, a resonant line tank circuit extending between said anodes, said circuit comprising a pair of hollow tubular transmission line conductors, one end of each of said conductors surrounding and conductively engaging one of said anodes, a pair of opposed metallic fins supported by the respective conductors, said fins extending axially substantially the length of said line and defining a gap therebe tween of substantially less width than the spacing between said conductors, and a conductive member adjustably bridging said gap.

3. In combination, in a push-pull amplifier for amplifying currents of a high frequency, the structure comprising a pair of similar thermionic amplifying devices disposed and arranged in parallel spaced relationship and having a pair of corresponding terminals, a transmission line having a pair of conductors terminating in conductive connections at said terminals and comprising elements of a resonant circuit therebetween adapted to have standing waves of current of said high frequency developed thereon, low resistance means positioned between said conductors and electrically associated therewith for materially decreasing the surge impedance of said line, said meansibeing in conductive engagement with said line along a substantial portion of its length and defining a gap between said conductors of less width than the spacing therebetween, and means to short-circuit said line substantiallyv at an antinode of said standing current waves.

4. A high frequency push-pull power amplifier structure comprising, in combination, a pair of electron discharge devices disposed and arranged in parallel spaced relation and each having an external anode base structure, a resonant line tank circuit between said anodes comprising a parallel pair of hollow, cylindrical, transmission line conductors, one end of each conductor conductively engaging the base structure of one of said devices, low resistance means positioned between said conductors and electrically associated therewith for materially decreasing the surge impedance of said line, said means being in conductive engagement with said line along a substantial portion of its length and defining a gap between said conductors of less width than the spacing therebetween, and a conductive member adjustably short-circuiting said line for adjusting the electrical length thereof substantially to resonance at the operating frequency of said amplifier.

5. The combination, in a high power push-pull amplifier structure adapted for operation at ultra high frequencies, of a pair of hollow, cylindrical, transmission line conductors disposed and arranged in parallel spaced relationship, a pair of thermionic devices each having a metallic base forming one electrode terminal thereof, said conductors engaging and supporting the bases of the respective devices and comprising elements of a resonant tank circuit therebetween, means to decrease the surge impedance of said line comprising a pair of opposed metal plates mounted adjacent each other between said conductors with one edge of each plate conductively engaging a respective conductor, opposing edges of said plates being in close proximity to each other and defining a gap of substantially less width than the spacing between said conductors, and an adjustable conductive member bridging said plates for adjusting the electrical length of said transmission line.

6. In combination, a push-pull power amplifier adapted for operation at ultra high frequencies, the structure comprising a pair of tubular conductors mounted vertically in parallel spaced relationship and comprising elements of a shortcircuited resonant transmission line, a pair of power amplifier tubes each having a metal base structure forming an anode terminal connection therefor, the upper ends of said conductors receiving and supporting said base structures, a pair of opposed, radial, metal flanges supported by the respective conductors, said flanges extending vertically substantially the length of said line and defining a gap therebetween of substantially less width than the spacing between said conductors and a metal member bridging said plates at the short-circuited end of said line.

7. A short-circuited resonant line structure for high frequency operation comprising a pair of spaced apart transmission line conductors adapted to have standing electrical waves of current and voltage developed thereon, low resistance means positioned between said conductors and electrically associated therewith for materially decreasing the surge impedance of said line, said means being in conductive engagement with said line along a substantial portion of its length and defining a gap between said conductors of less width than the spacing therebetween, and shortcircuiting means for determining the length of said line.

8. A resonant transmission line structure comprising a pair of spaced conductors, a pair of conductive plates mounted adjacent each other between said conductors with one edge of each plate conductively engaging a respective conductor, opposing edges of said plates being in close proximity to each other and defining a gap of substantially less width than the spacing between said conductors, and an adjustable conductive member bridging said transmission line for adjusting the electrical length of said line.

9. An open wire resonant transmission line for ultra high frequency operation comprising a pair of spaced apart transmission line conductors, means to decrease the surge impedance of the line comprising a pair of opposed flanges of high electrical conductivity respectively supported by said lines and extending inwardly toward each other, said flanges being separated to define a gap, and a shorting member conductively bridging said line.

10. A short-circuited resonant transmission line structure adapted to be energized to have standing current waves of a high frequency developed thereon comprising, in combination, a pair of spaced transmission line conductors, opposed metallic members conductively engaging the respective conductors and extending substantially the entire length of said line, said members defining a gap therebetween of substantially less width than the spacing between said conductors, and a metallic short-circuiting member bridging said gap substantially at an anti node of said high frequency standing current waves.

JAMES E. KEISTER. 

